I'm surprised this was left open. It seems way too broad to me; a lengthy answer could be written just about identifying rings, another about identifying potions, and so on.
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starsplusplusApr 30 '14 at 9:06

5 Answers
5

The most important thing to note is that identifying items in one game only helps you in that game. If ruby rings were rings of invisibility last game, but you died and started a new one, you know NOTHING about them. The exceptions are the trivial ones: potions of water and blank scrolls/spellbooks.

The second most important thing about identifying items is that each item can be blessed, uncursed, or cursed. If your role is Priest, then the blessed/uncursed/cursed status is automatically revealed to you for all items. If you are of any other role, then you need to utilize alternative methods to determine the status. The available options are:

You can figure out which items are which by dropping them on an altar (as long as you are not blind, so that you can see the flashes of light).

Reading a scroll of identify will also point out the status of the item.

As mentioned in Durathor's answer, pets will avoid walking on cursed items that are on the ground, only moving "reluctantly" if they ever stand on it. You can use this from the start to avoid an early demise from using cursed equipment.

Blessed items and uncursed items are much safer to use than cursed ones. Don't drink any cursed potions, read cursed scrolls, or put on any cursed rings/armor unless you know exactly what you are doing.

Rings and armour

Conversely, if you have rings or armor that you know is not cursed, you can put it on and take it off right afterwards. For certain types of equipment (ring of invisibility, gauntlets of dexterity, boots of levitation, etc) this may identify it immediately. Others will have clear effects, allowing you to #name them yourself. (Ring of conflict, gloves of fumbling, etc) This will also tell you how heavily enchanted a piece of armor is.
Note that certain types of rare headgear can still cause problems, even if they are not cursed. The helm of opposite alignment will lose you all your divine protection, and will block you from doing the quest. Anything that looks like a "conical hat" should be avoided unless you are a wizard, in which case you may want to wear the expensive type.

Potions

Potions provide you with some interesting options.

Clear potions are always water. However, they may be blessed (holy water) or cursed (unholy water), which have important uses. You can find out what kind of water you have by dropping the potion on an altar (they won't break, promise).

You can try #dipping a unicorn horn into a potion. Most bad potions will be neutralized this way. If you dip a unicorn horn into a brown potion, and the brown potion clears/changes color, don't drink any brown potions.
However, just because the potion did not change, does not mean it is safe to drink. Unicorn horns don't detect everything. (sleeping and acid, for example). Also, a potion of polymorph will cause you to lose your unicorn horn as it turns into something else. This can be prevented by dipping any other item into the potion first.

If you find a food shop, you can try dropping your potions there. Anything that the shopkeeper offers to buy from you is a "food potion." In practice, that means water, fruit juice, or booze.

Monsters will drink helpful potions (speed, healing, invisibility) and throw harmful potions (paralysis, blindness, acid) at you.
When in doubt, wait for this to happen instead of drinking random potions. The penalties are much less severe, especially if the potion was cursed.

If a potion was dropped by a nymph, and it's not one she stole from you, chances are that it's object detection.

The potion of oil is unique in that you can apply it to light it on fire like a lamp. No other potion is a valid target to apply.

Scrolls

For scrolls, I typically end up relying on price. As with all items, more expensive scrolls do 'bigger' things. Some specifics on scrolls:

First level of Sokoban always has two scrolls of earth in the same spot.

If you find a scroll in a one-square "closet", it will be a scroll of teleportation.

Scrolls of identify are really cheap. Cheaper than blank scrolls. This makes them pretty easy to identify.

I figured, since you already have the section about "The second most important thing", I'll append the full detail of BUC-detection to your answer rather than try to squeeze it into mine. If you object, feel free to rollback. ♪
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Grace Note♦Oct 12 '10 at 13:02

The most commonly known method is called "Price Identification", which abuses either proactive knowledge or reactive knowledge to identify items by seeing how they cost in a shop, either in selling or buying. Applying the same modifiers for your charisma, the same item will always cost the same amount. Proactively, this involves looking up tables of prices to know what you're looking at. Reactively, this involves remembering how much certain things cost as you go through multiple playthroughs. Note that price ID only gets you the price, and there are multiple items that will cost the same, so the point is mostly to narrow down what possible items it could be.

It should be noted that the Scroll of Identify is pretty much the cheapest scroll at 20.

Onto general strategies for specific items. Note that these won't help in all situations and there are some dangerous points to be careful of. I'm going to not cover what specific perils to watch out for, to give you some suspense.

Some wands can be identified by Engraving with them. This will either do nothing, produce a special message and/or effect based on the wand, or very loudly announce what the wand is. The first obviously doesn't help. The second won't directly identify the wand but it will help guide you to knowing what it is in order to zap it; note that some wands have matching phrases while others have unique ones (cold always has "some ice cubes fall out", while both sleep and death cause "bugs on the floor to stop moving"). The last one will treat it as if you had really identified it.

Scrolls, when read while confused, tend to do not quite what they're supposed to. However, a lot of the dangerous scrolls will not have deadly effects if read while confused, and some are actually beneficial. Reading a scroll counts as identifying it.

Rings can be dropped into sinks. Except for certain rings, this will destroy the ring in exchange for a message that will help identify the ring. So if you have a spare unidentified ring, try dropping it in a sink to tell what it is. This won't identify it directly, so you will have to name the item or otherwise remember which one you just handled.

I think a lot of the potion-based strategies are based on knowing the price before-hand, and then simply either quaffing them with less risk or Dipping items into the potion. That's very specific and I don't remember the whole lot off the top of my head. So barring that approach, note that a lot of the more dangerous potions will probably be thrown at you by enemies, while some of the really nice potions tend to be quaffed by enemies. Sometimes this will auto-ID the potion, while other times it'll prompt you to name the potion.

The obvious choice would be to use a scroll of identify (or better, learn the spell). Naturally, this requires knowing which scrolls are scrolls of identify, so back to square one.

The sad result of that is that some trial and error is necessary.

Now, there are ways to try things that are safer than others. For that, my best suggestion is use the item on someone else. Sadly this only works for potions, wands and (partially) scrolls. Wands are largely targeted, so simply choose your least favorite monster. Fewer scrolls are targeted, but those that are can likewise target a volunteer. Potions are fun because you get to throw them at your test subject. Of course, this method STILL has the chance to backfire, by using a good effect on a monster. You can choose mostly harmless monsters as your test subjects, but failing that, I still prefer speeding up a monster over the chance of disintegrating myself with my brand new wand.

One last thought, before doing your trial and error, you can try de-cursing the item in question, if possible. Cursed items are almost all bad, so if you can determine that an item is not cursed, dip it in holy water, or (I think this blesses the item?) offer a sacrifice with the item on the altar, before using the item, you're odds of bad things happening go way down.

On Armor & Weapons:

Putting on armor or wielding weapons sometime(?) identifies them, but this is risky due to not being able to remove cursed armor/weapons*. A trick to deal with this situation is to keep Scrolls of Destroy Armor/Weapon you find. (I originally thought they were useless!) Be sure to take off all your other armor before trying your new armor, to give the Destroy Armor scroll only one target.

*If I recall right, sometimes weapons are slippery instead? But that works out fine too.

Slipperiness comes from being covered in grease, such as from fried food. Cursed equipment will always be impossible to personally remove - but for example nymphs can steal your cloak right off of you.
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Grace Note♦Aug 13 '10 at 19:04

@Kelly No, they don't. It's a random choice, but many people will do things like drop everything they own except the cursed stuff and just waltz with a nymph, which will naturally give them no choice but the cursed items.
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Grace Note♦Aug 13 '10 at 19:10

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The slipperiness WillfulWizard mentions might be referring to the tendency of cursed darts/arrows/etc to 'slip' when thrown and go in a random direction.
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Larry WangAug 13 '10 at 19:17

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On the decursing point - I normally use the approach of dropping items and seeing if my pet will stand on them or not. If the pet won't stand on it, or only reluctantly, then it's generally cursed
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DurathorAug 19 '10 at 20:47